The present invention relates generally to imaging systems. More particularly, the present invention relates to the cooling of rotating anode x-ray tubes.
Electron beam generating devices, such as x-ray tubes and electron beam welders, operate in a high temperature environment. In an x-ray tube, for example, the primary electron beam generated by the cathode deposits a very large heat load in the anode target to the extent that the target glows red-hot in operation. Typically, less than 1% of the primary electron beam energy is converted into x-rays, while the balance is converted to thermal energy. This thermal energy from the hot target is radiated to other components within the vacuum vessel of the x-ray tube, and is removed from the vacuum vessel by a cooling fluid circulating over the exterior surface of the vacuum vessel. Additionally, some of the electrons back scatter from the target and impinge on other components within the vacuum vessel, causing additional heating of the x-ray tube. As a result of the high temperatures caused by this thermal energy, the x-ray tube components are subject to high thermal stresses which are problematic in the operation and reliability of the x-ray tube.
Typically, an x-ray beam generating device, referred to as an x-ray tube, comprises opposed electrodes enclosed within a cylindrical vacuum vessel. The vacuum vessel is typically fabricated from glass or metal, such as stainless steel, copper or a copper alloy. As mentioned above, the electrodes comprise the cathode assembly that is positioned at some distance from the target track of the rotating, disc-shaped anode assembly. Alternatively, such as in industrial applications, the anode may be stationary.
The target track, or impact zone, of the anode is generally fabricated from a refractory metal with a high atomic number, such as tungsten or tungsten alloy. A typical voltage difference of 60 kV to 140 kV is maintained between the cathode and anode assemblies to accelerate the electrons. The hot cathode filament emits thermal electrons that are accelerated across the potential difference, impacting the target zone of the anode at high velocity. A small fraction of the kinetic energy of the electrons is converted to high energy electromagnetic radiation, or x-rays, while the balance is contained in back scattered electrons or converted to heat. Ultimately, the back scattered electrons are absorbed by components within the vacuum vessel as heat energy. The x-rays are emitted in all directions, emanating from the focal spot, and may be directed out of the vacuum vessel.
In an x-ray tube having a metal vacuum vessel, for example, an x-ray transmissive window is fabricated into the metal vacuum vessel to allow the x-ray beam to exit at a desired location. After exiting the vacuum vessel, the x-rays are directed to penetrate an object, such as human anatomical parts for medical examination and diagnostic procedures. The x-rays transmitted through the object are intercepted by a detector and an image is formed of the internal anatomy. Further, industrial x-ray tubes may be used, for example, to inspect metal parts for cracks or to inspect the contents of luggage at airports.
Since the production of x-rays in an x-ray tube is by its nature a very inefficient process, the components in x-ray generating devices operate at elevated temperatures. For example, the temperature of the anode focal spot can run as high as about 2700.degree. C., while the temperature in the other parts of the anode may range up to about 1800.degree. C. Additionally, all of the components of a conventional x-ray tube insert must be able to withstand the high temperature exhaust processing when the vacuum vessel is evacuated, at temperatures that may exceed very high temperatures for a relatively long duration.
To cool the x-ray tube insert, the thermal energy generated during tube operation must be radiated from the anode to the vacuum vessel and be ultimately removed by a cooling fluid circulating over the exterior of the x-ray tube insert vacuum vessel. The vacuum vessel is typically enclosed in a casing filled with circulating, cooling fluid, such as dielectric oil. The casing supports and protects the x-ray tube and provides for attachment to a computed tomography (CT) system gantry or other structure. Also, the casing is lined with lead to provide stray radiation shielding. The cooling fluid often performs two duties: cooling the vacuum vessel, and providing high voltage insulation between the anode and cathode connections in the bipolar configuration.
Additionally, this conventional approach becomes even more problematic when combined with new techniques in x-ray computed tomography, such as fast helical scanning, that require vastly more x-ray flux than previous techniques. Due to the inherent poor efficiency of x-ray production, the increased x-ray flux is purchased at the expense of greatly increased heat load that must be dissipated. As the power of x-ray tubes continues to increase, novel cooling techniques must be developed to remove heat from the rotating anode structures.
Rotating anode x-ray tubes are used in mammography, vascular, and computed tomography x-ray systems. Rotating anode x-ray tubes are also ultimately limited in performance by their heat dissipation rate. The bearing components of the rotating anode typically have a temperature limit which is significantly less than the operating temperature of the rotating anode target. Typically, the rotating anode target operates at temperatures over 1000.degree. C. at the target ID. Consequently, the anode target must be thermally isolated from the bearing shaft by a long thermal barrier such that the temperature drop to the bearings closest to the heat source drops the temperature to below the bearing temperature design limit.
In a conventional rolling element x-ray tube bearing assembly, very little power is removed down the bearing shaft by design. If too much heat is allowed to go down the shaft, the temperature of the bearing races and solid lubricated ball bearings drastically increases and can exceed an acceptable limit. Such conditions lead to premature failure. Therefore, it is necessary to limit the maximum temperature in the bearings. Conversely, it is also desirable if more power could be transferred down the bearing shaft and out of the tube insert to aid in cooling the target. This would ultimately increase the power available from x-ray tube systems and, consequently, would provide greater subject (e.g., patient) throughput by the x-ray tube systems.
Another problem with conventional rotating anode x-ray tubes is that the internal diameter (ID) of the anode target can be extremely hot during operation, thereby reducing the strength of the anode material. This reduction in strength lowers the peak rotational operating speeds of the target. As a result, the peak power at which the x-ray tube can operate is reduced. The limit of anode rotational speed is caused by the peak temperatures under the electron beam. As the target spins faster, the local instantaneous heating under the electron beam is reduced.
Thus, there is a need for an improved method of dissipating heat from the anode of the x-ray tube. Further, there is a need for an x-ray tube which provides increased performance by more effective heat dissipation. Even further, there is a need for an x-ray tube which operates with a cooler anode, providing the capability of faster anode rotation and greater x-ray tube power.